Nope cause ps3 owners still have a ps3 and new consoles aren't released that soon.
Not all PS4 owners will have a PS3 too. What if somebody buys a PS4 and wants to play a highly revered PS3 game?
Streaming may work with fast internet connections but many dont have that luxury, especially here in India. And even if you do have a fast data pipe, what about the usage charges - they are going to be much higher than the price of the game itself I guess. Streaming might work in an ideal world, but to pass off streaming as a replacement for good 'ol native backward compatibility is just a business move with very questionable benefit to the end user.